Induction heated vapor source

ABSTRACT

Induction heated vapor source particularly suitable for vacuum deposition of thin films utililzing a flat wound work coil fabricated of a solid conductor enclosed in a hermetically sealed chamber. A cooling medium is circulated through the chamber about the coil, and the coolant is passed to and from the chamber through fittings which also serve to carry electric power to the coil. A crucible for holding the material to be vaporized is mounted on an adjustable pedestal outside the sealed chamber in the field produced by the coil.

BACKGROUND OF THE INVENTION

This invention pertains generally to apparatus for coating materialswith a vapor and more particularly to an induction heated source forvaporizing metals and other materials to be deposited in a vacuum.

Induction heated vapor sources heretofore provided for use in vacuumdeposition of thin films commonly have utilized heating coils fabricatedof a hollow conductor through which a coolant is circulated. The coil isenergized with a radio frequency current to produce a magnetic fieldwhich induces a secondary current in the load or succeptor thussupplying heat to the material, and the amount of heat produced isdependent upon both the amount of current and the number of turns in thecoil as well as the frequency of the current, nature of the material andthe relative geometry of the coil and load.

With heating coils fabricated of hollow conductors, the number of turnsthat can be used in a given space is generally limited, and consequentlya relatively high current is required to produce a given amount of heat.The higher current requires larger connecting cables and fittings, andwith large currents the connecting cables are frequently water cooledhollow conductors which are difficult to install and work with.

SUMMARY AND OBJECTS OF THE INVENTION

The vapor source of the invention utilizes a flat wound work coilfabricated of a solid conductor enclosed in a hermetically sealedchamber. A cooling medium is circulated through the chamber in contactwith the outside surface of the coil, and the coolant is passed to andfrom the chamber through fittings which also serve to carry electricpower to the coil. A crucible for holding the material to be vaporizedis mounted within the field of the coil on an adjustable pedestaloutside the sealed chamber.

It is in general an object of the invention to provide a new andimproved vapor source suitable for use in the deposition of thin filmsin a vacuum chamber.

Another object of the invention is to provide a new and improved vaporsource of the above character in which a given amount of heat isproduced with a minimum of current in a work coil of given size.

Another object of the invention is to provide a vapor source of theabove character utilizing a flat wound work coil of a solid conductor.

Another object of the invention is to provide a vapor source of theabove character in which the heating coil is enclosed in a hermeticallysealed chamber through which a cooling medium is circulated.

Additional objects and features of the invention will be apparent fromthe following description in which the preferred embodiment is set forthin detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of one embodiment of a vapor source accordingto the invention.

FIG. 2 is a cross sectional view taken along line 2--2 of FIG. 1.

FIG. 3 is a sectional view taken along line 3--3 of FIG. 2.

FIG. 4 is a sectional view taken along line 4--4 of FIG. 2.

FIG. 5 is an elevational view of the vapor source of FIG. 1 with thecoil housing removed.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The vapor source includes a base 10 which, in the embodimentillustrated, is fabricated of a suitable material such as sheet metaland formed to include a generally planar top wall 11, depending sidewalls 12 and 13, and a depending curved rear wall 14. A front wall 16extends between the lower portion of the side walls at the front of thebase, and an opening 17 is formed between the top, side and front walls.An additional opening 18 is formed in the side of the base between sidewall 12 and rear wall 14, and a portion of top wall 11 is cut away, asindicated at 19.

Surmounted on base 10 is a housing assembly 21 in which a hermeticallysealed chamber 22 is formed. The housing assembly is fabricated of ahigh temperature RF insulative material such as alumina or othersuitable ceramic, and it includes a cover 23 and a base 24. The coverincludes an inner side wall 26, an outer side wall 27, and annular topwall 28. Side walls 26 and 27 are generally cylindrical, and they aredisposed coaxially of each other. Housing base 24 includes a raisedcentral portion 31, in which an axial opening 32 is formed, and anannular outer portion 33 which serves as a bottom wall for chamber 22.Base 24 also includes an internally threaded cylindrical lip 34 in whichthe outer wall of the housing cover is threadedly received. O-rings 36and 37 extend about the inner and outer edges of base portion 33 to formhermetic seals with side walls 26 and 27 when cover 23 is tightened inbase 24.

A flat wound work coil 41 is mounted in the sealed chamber 22 withinhousing 21. This coil is fabricated of a generally flat, solid stripconductor, such as copper ribbon having a thickness of 0.085 inch and awidth of 0.285 inch. The conductor is covered by a suitable insulativematerial such as a fiberglass wrapped formvar coating. The coilcomprises a plurality of closely spaced windings wound about a verticalaxis 42, with the flat surfaces of the conductor facing axially of thecoil.

Means is provided for holding a material to be vaporized at apredetermined position with the work coil. This means includes acrucible 46 which rests on a pedestal 47 having a platform 48 and anaxially extending stem 49. This stem extends through opening 32 into theregion bounded by base 10 where it is accessible through opening 18.Annular spacers 51 are disposed coaxially of stem 49 between platform 48and the raised portion 31 of housing base 24 to support the crucible andpedestal in a desired axial position. The number and size of the spacersare chosen to position the crucible as desired.

In the preferred embodiment, crucible 46 is fabricated of a dielectricmaterial so that the energy from coil 41 will heat the material to bevaporized rather than the crucible. However, if desired, the cruciblecan be fabricated of any suitable material, including carbon, vitreouscarbon and other electrically conductive materials. Pedestal 47 andspacers 51 are fabricated of a high temperature insulative material suchas alumina or another suitable ceramic.

A cup-shaped heat shield 52 having an axial opening 53 is disposedcoaxially of crucible 46 between the crucible and the inner wall 26 ofhousing 21. The heat shield is preferably fabricated of a low densityceramic material such as a refractory metal oxide, and it serves toreflect heat radiated by the material in the crucible back to thecrucible. In addition, it provides means for containing the molten metalin the event that the crucible should fail.

Means is provided for circulating a cooling medium such as water throughchamber 22 to cool coil 41 and housing 21. This means includes fluidinlet and outlet feedthrough fittings 56, 57 which extend throughopenings 58, 59 in the bottom wall 33 of the housing. These fittings aresecured to the housing base by nuts 61, 62 and sealed by O-rings 63, 64.As will appear more fully hereinafter, inlet fitting 56 communicateswith the region of the chamber between coil 41 and inner housing wall26, and outlet fitting 57 communicates with the region between coil andouter housing wall 27 so that the coolant circulates fully about thecoil.

Means is provided for directing the circulation of coolant within thechamber. This means includes a cylindrical baffle 66 which is disposedeccentrically of inner housing wall 26, with inlet fitting 56 disposedwithin the baffle and outlet fitting 57 outside the baffle. The loweredge of baffle 66 engages the upper surface of bottom wall 33 and formsa generally fluid tight seal therewith. A generally annular baffle 67 ismounted on top of baffle 66 and engages the lower portion of coil 41.Baffle 67 is sealed to both baffle 66 and and coil 41, and the innerdiameter of baffle 67 is made larger than the diameter of wall 26 toform an annular passageway 68 between the baffle and this wall. An upperannular baffle 71 is mounted on the top of coil 41 and positioned toabut against the lower surface of top wall 28 when housing cover 21 istightened into housing base 24. Baffle 71 is formed to include aspirally extending groove 72 which cooperates with top wall 28 to form aflow passageway when the housing is assembled. The passageway formed bygroove 72 communicates with the regions inside and outside coil 41through openings 73 and 74 at the inner and outer ends of the groove.Baffles 66, 67 and 71 are fabricated of suitable insulative materialssuch as polypropolene and fiberglass.

A drain tube 76 fabricated of an electrically insulative material isconnected to outlet fitting 57 and extends through chamber 22 outsidecoil 41. The top of the drain tube abuts against the lower surface oftop wall 28, and a drain opening 77 is formed in the upper portion ofthe tube.

Feedthrough fittings 56 and 57 are fabricated of an electricallyconductive material, and the electrical connections to work coil 41 aremade through these fittings. At the lower end of the coil, the conductorwhich forms the coil passes through an opening in baffle 67 and isformed into a loop 79 which is affixed to the upper portion of fitting56. The conductor at the top end of the coil is twisted 90° and broughtback to the bottom of the coil in the form of an edge wound helicalwinding outside the main portion of the coil. At the bottom of the coil,the conductor from the top is formed into a loop 83 which is affixed tofitting 57. The helical return lead has been found to be effective ineliminating non-uniformities in the field which might be present with astraight return lead.

Electrically conductive terminals 86, 87 are connected to fittings 56,57 in base 10 and secured thereto by nuts 88, 89. These terminals aresupported in spaced relation from the top wall 11 of the base bystand-off insulators 91, 92, and the free ends of the terminals extendthrough opening 17 for connection to a source of RF energy.

Fluid connections to fittings 56, 57 are also provided in base 10. Inthis regard, elbow fittings 96, 97 are connected to the feedthroughfittings, and feedthrough connectors 98, 99 are mounted in the frontwall 16 of the base for connection to external flow lines. A tube 101fabricated of an electrically insulative material such as quartz orpyrex extends between fittings 86 and 96, and a short electricallyconductive tube 102 extends between fittings 87 and 97.

Operation and use of the vapor source can be described briefly. It isassumed that terminals 86, 87 are connected to a suitable source of RFenergy for energizing the work coil and that connectors 98, 99 areconnected to coolant inlet and return lines, respectively. The materialto be vaporized is placed in crucible 46 where it is heated andvaporized by the field produced by the energized coil. When the sourceis deenergized and cooled, the crucible can be removed by reachingthrough opening 18 in the base and lifting pedestal 47 by its stem.

The coolant from inlet connector 98 flows through feedthrough fitting 56and annular passageway 68 into the region of chamber 22 between coil 41and inner housing wall 26. At the top of this region, the fluid passesthrough opening 73 into the spiral passageway formed by groove 72, andfrom this passageway it flows through opening 74 to the region betweencoil 41 and outer housing wall 27. The spiral passageway imparts acircular flow to the coolant on both sides of the coil. The swirlingcoolant fills the region outside the coil and exits from the top of theregion through drain opening 77 in tube 76. From this tube, the coolantpasses through inlet fitting 57 and outlet connector 99 to the coolantreturn line.

The invention has a number of important features and advantages. Theflat wound work coil has about three times as many turns as aconventional heating coil of the same size fabricated of a hollowconductor, and the flat wound coil requires only about one-third as muchcurrent as a conventional coil for a given heat output. The smallercurrent requirement permits the use of smaller connecting cables andfittings which are relatively economical and easy to use. The coil isenclosed in a hermetically sealed housing, and the source isparticularly suitable for use in a high vacuum chamber.

It is apparent from the foregoing that a new and improved vapor sourcehas been provided. While only the presently preferred embodiment hasbeen described, as will be apparent to those familiar with the art,certain changes and modifications can be made without departing from thescope of the invention as defined by the following claims.

We claim:
 1. In an induction heated source for vaporizing a material:A.a base; B. a helically wound work coil surmounted on the base andcomprising a plurality of closely spaced windings of a generally flat,solid electrical conductor oriented with its flat surfaces facingaxially of the coil; C. a housing having radially spaced apart inner andouter side walls disposed coaxially of the coil, said housing and coilforming an inner chamber between the inner periphery of the coil and theinner wall of the housing and an outer chamber between the outerperiphery and the outer wall of the housing; D. means for supporting thematerial to be vaporized in a predetermined position in the regiondefined by the inner wall of the housing; E. a flow inlet communicatingwith the inner chamber and adapted for connection to a source of liquidcoolant; F. means forming a spirally extending flow passageway betweenthe inner and outer chambers at one end of the coil for carrying thecoolant from the inner chamber to the outer chamber and imparting aswirling movement to the coolant; and G. a flow outlet communicatingwith the outer chamber for passing the coolant from the chamber.
 2. Thesource of claim 1 wherein the means for supporting the material to bevaporized includesA. a crucible for holding the material; B. an axiallymovable pedestal having a platform on which the crucible rests and anaxial stem depending from the platform; C. a spacer disposed coaxiallyof the stem between the base and the platform for supporting thecrucible and pedestal in a predetermined position.
 3. The source ofclaim 2 wherein the stem extends into the region of the base and thebase has a cut-away portion permitting access to the lower portion ofthe stem for raising the pedestal and crucible from the predeterminedaxial position.
 4. The source of claim 2 further including a heat shielddisposed between the crucible and the inner wall of the housing.
 5. Thesource of claim 1 wherein the flow inlet and outlet include electricallyconductive members through which electrical connections are made to thecoil.
 6. The source of claim 1 wherein first and second electricalconnectors extend through the lower wall of the housing, the lower endof the coil is connected to the first connector, and a conductor extendshelically in the outer chamber from the upper end of the coil to thesecond connector.
 7. In an induction heated source for vaporizing amaterial:A. a base; B. a housing mounted on the base having generallycylindrical inner and outer side walls disposed coaxially of a generallyvertical axis and annular top and bottom walls extending between theside walls to form a hermetically sealed chamber; C. an electricallyenergizable work coil wound about the generally vertical axis anddisposed between the side walls of the housing, said coil comprising aplurality of closely spaced windings of a generally flat, solidconductor oriented with the flat surfaces of the conductor facingaxially; D. a crucible disposed coaxially of the coil and within theinner wall of the housing for holding the material to be vaporized; E.an axially movable pedestal supporting the crucible; F. means forholding the platform and crucible in a predetermined axial position; G.fluid inlet and outlet means communicating with the chamber forcirculating a cooling medium about the coil, said fluid inlet meanscommunicating with the chamber in a first region between the coil andone of the side walls of the housing and said fluid outlet meanscommunicating with the chamber in a second region between the coil andthe other side wall; and H. means forming a spirally extending flowpassageway at one end of the coil for carrying the cooling mediumbetween the first and second regions.
 8. The source of claim 7 furtherincluding a heat shield disposed between the crucible and the inner wallof the housing.